Effects of hypoxia and low pH on mosquito insecticide toxicity in two commercially important shellfish species

Organophosphate and pyrethroid insecticides are commonly used to control mosquito populations. Concerns surrounding the use of these insecticides in the coastal zone include toxicity to non-target aquatic species and a lack of knowledge on the combined effects of insecticide toxicity and abiotic stressors on estuarine organisms. Information about multistressor effects is especially needed for economically and ecologically important shellfish species. To determine the effects of mosquito control insecticides on bivalve molluscs, mortality was determined at 4 d and 21 d for the larval and juvenile stages of the Eastern oyster, Crassostrea virginica, and the northern hard clam Mercenaria mercenaria, using two pyrethroids, resmethrin and permethrin, and an organophosphate, naled. Insecticide effects on swimming behavior were quantified after 4 d exposure in larval oysters. Sublethal effects on juvenile clam and oyster growth were also assessed after 21 d insecticide exposure. Overall, clams were more susceptible to mortality from acute exposures than oysters. For oysters, naled was the most toxic compound in larvae, while for clams; resmethrin was the most toxic compound. Mortality for both species generally increased with chronic insecticide exposure. Insecticide exposure also caused sublethal effects, including decreased swimming activity and growth. To determine whether abiotic stressors such as pH (lowered by hypercapnia) and low dissolved oxygen modified insecticide toxicity, acute tests were conducted with hypoxia (20% air saturation) and hypercapnia (approximately pH 7), with and without resmethrin exposure. Hypoxia, hypercapnia and a combination of hypoxia and hypercapnia caused mortality in larval clams and increased resmethrin toxicity. The insecticide concentrations determined to cause mortality and decrease growth in clams and oysters (> 0.5 mg/L) were considerably higher than concentrations measured in the environment (> 0.0005 mg/L). These data will benefit both shellfish aquaculture operations and environmental resource agencies as they manage the use of mosquito control insecticides near coastal ecosystems.